The fabrication of transparent, polycrystalline ceramic articles such as magnesium aluminate (spinel) and aluminum oxynitride (ALON), yttrium oxide, etc. by hot pressing and pressureless sintering has long been known and used as prior art. For example, over 50 patents exist that describe in considerable depth the methods used to produce transparent spinel by either or both hot pressing and pressureless sintering methods. Many of these patents include hot isostatic pressing (HIPing) as the final step needed to achieve near theoretical density and the maximum in-line transmission over the transmission range from 0.2 to 6 μm.
There is, however, very little prior art specifically addressing the characterization of these articles prior to final polish. Because the primary applications for these materials specifically target their broad-spectrum transmittance of electromagnetic (E-M) waves, such characterization must also primarily focus on the E-M properties of the material. Given the imaging applications for these articles, of particular interest are transparency variations (such as haze) and material discontinuities (such as solid or agglomerated inclusions, voids or localized phase differences) with a size on the order of 1 mm or less that degrade the transmission and overall optical quality of the finished, transparent ceramic articles, as well as overall non-uniformity of dielectric properties (i.e., refractive index). The most common methods used for the nondestructive evaluation of ceramic materials are ultrasonic reflectometry (in which the magnitude and timing of reflected or transmitted ultrasonic acoustic pulses are used to interrogate the material uniformity and/or variability) and x-ray radiography (in which material inhomogeneities are detected by their differing radiographic density). However, these methods either are not electromagnetic in nature (ultrasound) or interact with the material at a frequency and with a wavelength dramatically different than that of the spectrum and feature sizes of interest, respectively.